Cavity resonator



Nov. 16, 1954 T. 'r. PUREKA CAVITY RESONATOR :5 Sheeis-Sheet 1 FiledJuly 31, 1951 IN V EN TOR.

THOMAS T. PUREKA Nov. 16, 1954 T. T. PUREKA 2,694,795

CAVITY RESONATOR Filed July 31, 1951 3 Sheets-Sheet 2 IN V EN TOR.

THOMAS T. PUREKA rfffrney NOV. 16, 1954 T, PUREKA 2,694,795

CAVITY RESONATOR Filed July 31, 1951 3 Sheets-Sheet 5 7/2 Qdllm mgUnited States Patent CAVITY RESONATOR Thomas T. Pureka, Lynn, Mass.,assignor to the United States of America as represented by the Secretaryof the Army Application July 31, 1951, Serial No. 239,484

3 Claims. (Cl. 333-83) greater, use is made of reflexvelocity-modulation tubes.

An example of the latter is a IO-cm. oscillator identified in the tradeas a Type 707A McNallytube. These tubes employ cavities as the resonantelement which determines the frequency. A cavity resonator in thisspecific application of the same can be considered simply as a parallelresonant circuit placing an alternating voltage between two grids in thetube through which the electrons must pass.

Since there are wide variations between different operating conditionsand between different tubes, such as in I the respective frequencies andoutputs of the latter, it is essential that the associated mechanicaltuning be adjusted so that the operating point falls at a point in themode where the frequency can be shifted a reasonable amount in eitherdirection with the electrical control without too much change in outputpower.

In the prior practice of using a wave guide for the resonant circuit ofa velocity-modulation oscillator, there has been a practical limitationof two-to-one in the frequency range of the cavity because of wave-guidecut-off on the low-frequency end and the occurrence of multiple modes onthe high-frequency end. In the various structures proposed heretoforefor the purpose of avoiding such limitation, an example of which isshown in Patent No. 2,410,109, issued October 29, 1946, to John C.Schelleng, the improvement obtained in operating characteristics hasonly been of minor consequence. The reason for this can be attributed tothe fact that in none of the designs of the prior art are both the widthand length of the cavity changed simultaneously.

With the foregoing in mind, it is one of the objects of the presentinvention to provide an improved cavity resonator of the characterreferred to which has advantages over the various constructions proposedheretofore in the way of a substantial increase in frequency rangepossible and which, at the same time, is relatively simple inconstruction and manner of operation.

Other objects and advantages will hereinafter appear.

In accordance with the invention, adjustment of frequency range with acavity resonator is accomplished by changing both width and length ofthe cavity simultaneously.

For the purpose of illustrating the invention, an embodiment thereof isshown in the drawing, wherein Fig. 1 is an elevational view, partly insection, of a cavity resonator having structural and operatingcharacteristics in accordance with the present invention and mounted ona velocity-modulation tube of a conventional type, the section beingtaken on the line 1--1 in Fig. 2;

Fig. 2 is a plan view partly in section, the section being taken on theline 22 in Fig. 1;

Fig. 3 is a simplified view comparable to Fig. 2 and illustrative of theoperating action in Figs. 1 and 2;

Fig. 4 is a view similar to Fig. 1, partly in section and illustrativeof a modification, the section being taken on the line 4--4 in Fig. 5;

Fig. 5 is a plan view partly in section, the section being taken on theline 5-5 in Fig. 4; and

Fig. 6 is a simplified view comparable to Fig. 5 and illustrative of theoperating action in Figs. 4 and 5.

In Fig. 1 reference numeral 10 designates a conventional reflexvelocity-modulation oscillator of the Mc- Nally type aforesaid. Aboutthe tube gap and the usual cavity grids (not shown), is disposed aresonant cavity 11 embodying the present invention.

Cavity 11 comprises four driven members or parts 12, 14, 15 and 16complementary to which, and slideable with respect to which, are fourcontact plates or follower parts 17, 18, 19 and 20. These eight parts,engaged and disposed as shown and together with top and bottom plates orcasing parts 21 and 22, respectively, provide a cavity 23 about the gapof tube 10. The outer cylindrical wall or surface of tube 10 at the gapthereof is represented by the broken line 10a in Figs. 2 and 3.

It is well known that if the total volume of a cavity such as 23 isincreased, the frequency of operation is decreased, and vice versa. Fora detail treatise on the basic requirements and theory of operation ofcavity resonators for the same or similar purposes as resonant cavity11, reference is made to volumes 14 and 16 of Radiation LaboratorySeries published 1948 by McGraw- Hill Book Company, Inc., of New York,N. Y., and to Principles of Radar copyrighted 1944 by The TechnologyPress, Massachusetts Institute of Technology, Cambridge, Massachusetts.

Integral with each of the parts 12, 14, 15 and 16 are the respectivesimilar racks 24, each of the latter being guided by and having a snugbut sliding fit between plates 21 and 22. In mesh with each rack 24 is apinion 25 journaled, as shown, in casing 21, 22 and integral or fixedfor rotation with a gear 26.

In mounting cavity 11 on tube 10, a clamping ring 26 is first slippedover the same to about the position shown in Fig. l; a split, flexiblewasher 27 is forced over the usual contact discs 28 and 29; the casingassembly or unit is placed on the tube; a second, split, flexible washer30 is placed against disc 28; and a second clamping ring 31 is screwedinto plate 21. By tightening rings 26 and 31, the contact discs 28 and29 are clamped tightly to the respective top and bottom plates of thecavity, for good radio-frequency contact.

The electrical characteristics of cavity 23 which is defined by theefiective or exposed surface portions of parts 12, 14, 15, 16, 17, 18,19 and as well as by such surface portions of plates 21 and 22, aresimilar to those of a fixed cavity. For example, the resonant circuit issuch that the maximum E field will appear across the tube gap.

Mounted on the top plate 21 and rotatable with respect thereto about thetube axis, is a frequency-adjustment knob or dial 32 the gear teeth 33of which mesh with gears 26.

Upon rotation of knob 32 the identical gears 26 and the identicalpinions integral or fixed with respect to the latter, are rotatedsimultaneously and in the same direction. Straight-line movement isthereby imparted to racks 24 and the associated members 12, 14, 15 and16, but in different directions each of which is perpendicular to thetube axis. If knob 32 is rotated counterclockwise, as viewed in Figs. 2and 3, members 12, 14, 15 and 16 will recede simultaneously and by thesame amount measured radially from the tube axis. Such action will beaccompanied by similar and simultaneous recession of the associatedfollowers or contact plates 17, 18, 19, and 20 and further andcorresponding compression of the coil springs 34 which maintain goodcontact between the sliding surfaces of all parts. In Fig. 3 the variousparts are shown receded to a position whereat cavity 23 is expanded, thetotal volume being substantially greater than that in Figs. 1 and 2.

To facilitate assembly, the casing has been shown as being comprised ofthe two plates or sections 21 and 22 which may be welded, cemented orotherwise secured together at the respective adjacent edges of the same,i. e., along the line 21a. For the same purpose, one or both of thecasing sections 21 and 22 may be made up of two or more parts suitablysecured together after assembly. All parts are made of material having arelatively high coeificient of electrical conductivity.

Figs. 4, 5 and 6 illustrate a modification or alternative form whereinthere are but two simultaneously receding driven members 12a and 15a andbut two associated followers or contact plates 17a and 19a. In all otherrespects, the structure and operating action of the various parts aresubstantially the same as in Figs. 1, 2 and 3.

For purposes of calibration, an appropriate scale may be engraved orotherwise placed on the beveled edge of the frequency-adjustment knob ordial 32'. It is also proposed to employ a set-screw or other suitablemeans to hold knob 32 against accidental rotary movement from anyposition of adjustment.v For example, arubber band or split ring .35 maybe inserted between knob 3.2. and the central cylindrical portion ofplate 21 on which the knob. is mounted, The grip or spring-action ofband 35 is made sutficient to hold the various parts. against creepagesuch as might be caused by the constant and inwardly-directedforceexerted on each rack 24 by the associated spring 34.

It will be understood that various other modifications, such as in thesize, shape and arrangement of the parts, are possible without departingfrom the spirit of the invention or "the scope of theclaims.

What is claimed is:

1. A cavityresonator comprising a plurality of movable contiguousdri-ver and follower walls surrounding and symmetrical about a given.longitudinal axis, said walls. being alternately disposed with. respectto each other, each of said walls having a sliding contact with thewalls. adjacent thereto, whereby said walls define said cavityresonator; a housing for said driver and said follower walls, each ofsaid follower walls including a longitudinal member disposed within akeyway therefor in said housing, a spring disposedbetweenone end of eachlongitudinal member and the endof its respective keyway to maintain saidfollower walls in positive contact with said driver walls in allpositions of said driver walls, each of said driver walls includingarack portion disposed withina keyway therefor in said'housing; andmeansfor simultaneously-moving alla of said walls comprising a pinionmeshed withtthe rack. portion. of each of said driver parts, gear means.attached toeach of said pinions and a toroidallyshaped knob. havingageared surface, said gear means beingv meshed with said geared surfaceof said knob.

2. A cavity resonator, asdefined. in claim 1, wherein there are atleastfourdriver walls disposed. at 90 intervals. aboutsaidilongitudinal. axis, each: of said driver walls including-anisosceles. right-triangular portion disposed; within said; housing, thejunction of the legs of said triangularportion. being attached to. oneend of said rack portion, and wherein there are at least four followerwalls, each of which is symmetrically disposed between each of saiddriver walls, each of said follower walls including a contact platedisposed within said housing and attached to the other end of saidlongitudinal member, said contact plate being spring-pressed intocontact with a pair of colinear legs of adjacent triangles, said legs ofsaid triangular portions being no more than one half the length of saidcontact plate when the distance between said driver walls and saidlongitudinal axis is at its minimum limit.

3. A cavity resonator as defined in claim 1, wherein there are at leasttwo driver walls disposed at 180 intervals about said longitudinal axis,each of said drivers including an isosceles right triangular portiondisposed within said housing with the junction legs of said triangularportion attached to one end of said rack portion, and wherein there areat least two follower walls each of which is symmetrically disposedbetween each of said driver walls, each of said follower walls includinga longitudinal member disposed within said keyway .with a wall sectionattached to the other end of said longitudinal member, said wall sectionbeing perpendicular to said longitudinal. member and having twooppositely disposed contact sections, each of said contact sectionsatfixed to an end of said wall section. at a 135 angle whereby saidcontact sections are spring-pressed into contact with adjacent legs ofsaid two triangularportions, saidlegs of said triangular portionbeing,at most, the same length as said contact sections.

References Cited in the file of this patent UNITED STATES PATENTS 62Great-Britain Jan. 7', 1857

